A number of systems for tracking vehicles, such as aircraft, using time difference of arrival (TDOA) techniques have been developed. The following U.S. Patents and Patent Applications, all incorporated herein by reference, disclose various aspects of TDOA tracking techniques:                U.S. patent application Ser. No. 11/545,800, filed Oct. 10, 2006;        U.S. patent application Ser. No. 11/492,711, filed Jul. 25, 2006;        U.S. patent application Ser. No. 11/429,926, filed on May 8, 2006;        U.S. patent application Ser. No. 11/343,079, filed on Jan. 30, 2006;        U.S. patent application Ser. No. 11/342,289 filed Jan. 28, 2006;        U.S. patent application Ser. No. 11/209,030, filed on Aug. 22, 2005;        U.S. patent application Ser. No. 11/257,416, filed on Oct. 24, 2005;        U.S. patent application Ser. No. 11/203,823 filed Aug. 15, 2005;        U.S. patent application Ser. No. 11/145,170 filed on Jun. 6, 2006;        U.S. patent application Ser. No. 10/743,042 filed Dec. 23, 2003;        U.S. patent application Ser. No. 10/638,524 filed Aug. 12, 2003;        U.S. patent application Ser. No. 09/516,215 filed Feb. 29, 2000;        U.S. patent application Ser. No. 60/123,170 filed Mar. 5, 1999;        U.S. patent application Ser. No. 10/319,725 filed Dec. 16, 2002;        Provisional U.S. Patent Application Ser. No. 60/440,618 filed Jan. 17, 2003.        
In a Time Difference of Arrival tracking system, a radio signal (either generated by, or reflected from a target) may be received from the target at a number of receiving stations (e.g., three or more) and the time the signal (time stamp) is received, carefully measured at the receiving station. This time information for the signal may then be sent to a central processor which receives the time stamps from the various receiving stations, and from that data, can calculate with some precision, the location of the target (e.g., aircraft).
Accuracy of the system relies in part on the accuracy of the time-stamping technique. Each receiver has to have an accurate clock, or at least be accurate relative to the clocks of the other receivers. In the past, various solutions have been tried to provide the most accurate clock synchronization between receivers. For example, some systems utilize GPS clock signals from satellites in order to synchronize clocks between various receivers. Others may utilize a beacon with a known position, transmitting signals to various receivers. The receivers may then synchronize their local clocks, based on the apparent arrival time of the signal and the known position of the signal source.
However, there may be situations where a target or beacon of known position is not available for synchronizing remote clocks. For example, in a portable system, the location and distance of a known beacon may not be readily available. Alternately, there may be situations where master clock signals, such as so-called lighthouse beacons or GPS signals, are lost. In such a situation, position data for aircraft may be inaccurate until the clock signals can be re-established. Thus, a need exists in the Prior Art for a system whereby remote clocks can be synchronized without the need for a beacon or GPS signal, to provide accurate tracking for aircraft.